The Effect of the Fracture Process on the Strength of Composite Laminates.

The role of the fracture process will be demonstrated by examining failure by two specific phenomena. The first is failure caused by a hole where carbon/epoxy laminate strength is diminished due to the hole. Local strain concentration occurs at the sides of the hole due to redistribution of the load from the cut fibers. The load in the cut fibers can be transferred directly into the adjacent continuous fibers through in-plane shear. Load can also be transferred to the adjacent angle ply by interlaminar shear. When the capability of the matrix is exceeded, matrix failure occurs in the form of longitudinal splits and delaminations which redistribute load away from the proximity of the hole, reducing the strain concentration in the continuous fibers. The second phenomenon used to demonstrate the role of the fracture process involves fatigue. The fatigue life of carbon/epoxy laminates under simple tension loading is diminished when the laminate contains off-axis plies. Local strain concentration in the zero degree ply is created in the vicinity of a crack tip when a transverse crack develops in the off-axis ply. Both mechanically applied strains and residual curing/thermal strains must be considered when computing transverse strain. It is the purpose of this paper to demonstrate that laminate rupture occurs when the strength of the zero fiber is exceeded, recognizing the local state of strain resulting from matrix fracture. This demonstration will be accomplished by presenting analytical results which model specific increments of the fracture process as determined from X-ray images of test coupons. Both splitting and delamination at a hole and transverse splitting in a laminate without a hole are studied. The relationship between zero degree fiber local strain and matrix fracture is established for: 1) one specific hole size and three specific laminate constructions and 2) 90° plies in between two zero ply layers. This work strives to yield understanding as opposed to design procedures.